The optical distance measuring systems used in present day arrangements for making linear measurements as a rule operate according to the triangulation principle, that is, these systems include a light source which directs a measuring beam onto the object to be measured and a receiver optic is mounted so that its axis is inclined with respect to the beam axis. These distance measuring systems are often referred to as "optical measuring heads" and laser diodes are primarily used as beam sources therein. For receivers in such systems, either position-sensitive photodiodes connected to analog signal processors or receiver lines are used. The receiver lines can comprise a plurality of individual elements arranged one next to the other and these can be, for example, diode arrays or so-called CCD-arrays.
Especially with respect to receivers of the first-mentioned type, the problem occurs that the measurement precision is dependent upon the intensity of the measuring light which the receiver receives back from the surface of the workpiece. Since the intensity of the received light is however very dependent on the condition of the surface of the workpiece to be measured, an attempt must be made to maintain the level of the received measuring light constant via a control of the intensity of the beam source. Optical measuring heads which have such a control arrangement are disclosed, for example, in the following: German published patent applications No. 3,147,129 and No. 3,119,505; German Patent No. 3,151,800; and, U.S. Pat. Nos. 3,612,890 and 4,375,921.
In the known measuring heads, the control is achieved in that the intensity of the light emitted by the beam source is controlled either via the current of the light source or the transmittance of an electro-optic modulator connected ahead of the light source.
However, and especially when lasers are used as light sources, this kind of control has its limits with respect to the dynamic range is attainable. These limits are imposed since the non-linear current/power characteristic of laser diodes permits only slight deviations from the optimal operating point at which the laser diode is driven. Furthermore, the radiating characteristic of the laser diode changes and therefore the radiation profile changes with a change in the operating current. In the lower power range, the laser diodes furthermore have a high background component of spontaneous emission when compared to gas lasers. The two last-mentioned effects likewise act negatively on the measurement precision when current control is utilized.
It is further known to modulate the light sources in the measuring heads mentioned above. For this purpose, the duration of a cycle of the frequency is however constant since the modulation of the light source functions only to suppress constant disturbance light levels which otherwise would falsify the measuring result. This suppression is performed in combination with a so called phase sensitive detector (psd) tuned to the modulation frequency.